A titanium alloy has a density of about 60% of steel and shows a high relative strength (=tensile strength/density), so is being used for machine parts for which lighter weight is demanded such as bolts, engine valves, and connecting rods.
As typical α+β-type titanium alloys which are used for such machine parts, Ti-6 mass % Al-4 mass % V (hereinafter referred to as “Ti-6Al-4V”) and Ti-3 mass % Al-2.5 mass % V (hereinafter “Ti-3Al-2.5V”) are used.
The shape of these machine parts is in general similar to that of rods which have long axes (except large ends and small ends of connecting rods). Force is likely to be applied in the long axis direction, so rigidity in the long axis direction is particularly demanded.
To raise the rigidity of a machine part, it is sufficient to increase the cross-sectional area of that part. However, if just increasing the cross-sectional area, the mass of the machine part increases. To raise the rigidity without increase in the mass of the machine part, it is necessary to increase the Young's modulus of the material itself.
The Young's modulus of titanium (at room temperature) is 88 to 113 GPa (9,000 to 11,500 kgf/mm2) which is as a small value as about a half that of a ferrous material, so titanium with high Young's modulus is highly needed.
Further, in addition to the above machine parts, in general, metallic materials with a high Young's modulus are desired for applications for machine parts of motorcycles, cars, and bicycles.
Titanium consists of the α-phase which is comprised of the hexagonal close-packed lattice (hereinafter “hcp”) and the β-phase which is comprised of the body centered cubic lattice (hereinafter “bcc”). The Young's modulus of the α-phase is around 110 GPa, while that of the β-phase is around 90 GPa. The α-phase has about 20% higher Young's modulus than the β-phase.
For this reason, as explained above, α+β-type titanium alloys, mainly comprised of the α-phase, are being used for bolts, connecting rods, engine valves, etc. for which high rigidity is demanded.
As a method for increasing the Young's modulus of a titanium alloy, for example, there is the method of adding B (boron) to the titanium alloy and causing metal borides with high Young's moduli to be dispersed so as to raise the rigidity (for example, PLT 1).
In addition, there is the method of making a composite of high Young's modulus SiC fibers or carbon fibers with a titanium alloy (composite material).
For production of bolts, engine valves, and connecting rods made of a titanium alloy, round bars which are produced by hot working are being used as materials.
Bolts are produced by hot or cold forging or rolling, or cutting a round bar material.
The method of production of an engine valve includes an upset method consisting of heating an edge part of the round bar material to form an umbrella-like part and a hot extrusion method in which a round bar material is hot-extruded.
Connecting rods are produced by hot forging a round bar material.
In the above way, machine parts made of a titanium alloy are mainly being produced using round bars made of the titanium alloy as materials.
It is known that the hcp titanium α-phase has crystal orientation anisotropy in Young's modulus. The crystal orientation of the longitudinal direction of the round bar used as a material has a large influence on Young's modulus.
In a high strength titanium alloy columnar shape (round bar) which is produced by hot rolling and is the material for cold forging, crystal orientations in which c-axes of hcp are aligned in the circumferential direction or radial direction of the columnar shape are accumulated. Further, it is learned that the X-ray diffraction intensity from (0002) basal plane of the hcp measured on the T-cross-section of the columnar shape is extremely low and that the c-axes of the hcp are not accumulated in the longitudinal direction of the columnar shape (PLT 2).